1. Field of the Invention
The present invention relates to a valve timing control system for an internal combustion engine, and more particularly to a valve timing control system for an internal combustion engine which adjusts opening and closing timing (valve timing) of an intake valve and/or an exhaust valve according to operating state of the internal combustion engine.
2. Description of the Related Art
The conventional techniques in this field include ones disclosed in, for example, Japanese Patent Application Laid-open no. Hei 7-91280 and Japanese Patent Application Laid-open no. Hei 10-227235. These publications indicate techniques for obtaining desired valve timings by means of valve timing control mechanisms, which use oil pressure-type actuators. Also disclosed are examples in which, in these valve timing control mechanisms, measures are taken to resolve a control deviation in valve timing control caused by a difference in a viscosity state of oil at each oil temperature.
In accordance with the above-mentioned conventional techniques, in order to perform a correction of a control amount, in the former technique, there is arranged an oil temperature sensor for detecting an oil temperature, which causes a cost increase. Further, in the latter technique, the temperature of cooling water at the starting time, and an operating state of the internal combustion engine (for example, a heat generation amount), are used to estimate the oil temperature. However, after the internal combustion engine has been stopped, the water temperature drops faster than the oil temperature, and when the engine is restarted after being left off for a certain amount of time, the difference between the water temperature and the oil temperature is great, and thus oil temperature cannot be estimated precisely. In a case where the cooling water temperature is substituted for the oil temperature immediately after starting the engine, an appropriate control amount cannot be given, and thus there was a problem that a difference between a target valve timing and an actual valve timing was great.
Further, there also is a conventional device, which has a lock mechanism and performs a lock pin release operation, where the oil temperature is estimated from the water temperature and then the control amount is determined. However, as described above, since the precision of the oil temperature estimation immediately after starting the engine is poor, there is set a control amount that includes an extra amount so that the lock pin is removed without fail. In this device, unnecessary pin-pulling operations are performed even after the lock pin release. Thus, a delay occurred in following up the actual target timing normally required for the operating state, and there was a problem that this deteriorated drivability of the internal combustion engine.
The present invention has been made to solve the above-mentioned problems, and therefore has an object thereof to obtain a valve timing control system for an internal combustion engine in which precision in estimating an oil temperature at a starting time is improved and a control deviation is reduced by giving an appropriate control amount.
A valve timing control system for an internal combustion engine according to the present invention, comprises: operating state detecting means for detecting an operating state of an internal combustion engine; actual valve timing detection means for detecting valve timing of at least one of an intake valve and an exhaust valve; target valve timing setting means for setting a target valve timing for the valve timing, based on a detection result from the operating state detecting means; an actuator for changing the valve timing of at least one of the intake valve and the exhaust valve; oil pressure adjusting means for supplying oil to the actuator to drive the actuator and performing adjustment of the oil pressure thereof; actual valve timing control means for controlling the actuator by controlling the oil pressure adjusting means, in order to make the valve timing follow up the target valve timing; and oil temperature estimation means for estimating the temperature of the oil supplied to the actuator by the oil pressure adjusting means, based on a state of the internal combustion engine during its previous operation time, and based on its current operation state, wherein the actual valve timing control means switches a control amount for controlling the oil pressure adjusting means, based on the oil temperature estimated by the oil temperature estimation means.
Therefore, the actual valve timing can be accurately controlled with respect to the target valve timing, to thereby obtain an effect that deterioration of the drivability, fuel efficiency, and exhaust can be prevented.